There is a significant effort taking place in many parts of the world to produce transportation fuels, particularly gasoline and diesel fuels, from energy sources other than petroleum. For example, research and development in Fischer-Tropsch technology has been on-going for several decades to produce diesel fuels from syngas derived from natural gas and coal. More recently, there is a significant effort taking place to convert renewable resources, such as biomass and triglycerides to transportation fuels. Before biomass can be converted to a transportation fuel via the Fischer Tropsch or similar process (synthetic fuel, or synfuel), it must first be converted to a syngas comprised primarily of CO and H2, which can then be sent to downstream processing to produce various chemical and transportation fuel products. Conversion of biomass to syngas is typically accomplished by gasification that converts the biomass into predominantly carbon monoxide and hydrogen (syngas) by reacting the carbonaceous material of the biomass at high temperatures with a controlled amount of oxygen and/or steam. The resulting syngas can be, inter alia, burned directly in internal combustion engines, used to produce methanol and hydrogen, or methanol and dimethyl ether, or converted via the Fischer-Tropsch process into synthetic fuels.
Syngas produced from biomass has a different characteristic composition than syngas produced from coal or natural gas because of differences in the heating value and chemical composition of biomass, coal, and natural gas. Specifically, syngas produced from biomass has a significantly lower H2/CO ratio than syngas produced from coal or natural gas. As such, processes originally developed to convert high ratio (H2/CO>2) syngas from natural gas or coal will be inefficient when applied to biomass. One way to overcome this problem is to “shift” the ratio of syngas produced from biomass to a higher ratio via the water-gas shift reaction. This can result in higher ratio syngas, but can be thermally inefficient due to the requirement to produce steam and result in lower carbon yield from the process. For this reason, it is desirable to develop new processes that can efficiently utilize low ratio syngas produce from biomass or other carbonaceous feed stocks with low heating value.
There are two well established processes to convert syngas to liquid transportation fuels. The Fischer Tropsch process converts syngas to diesel fuel and utilizes syngas with a ratio greater than 2:1 H2/CO. The MTG, or methanol to gasoline process, produces gasoline from syngas via a methanol intermediate. Production of methanol also requires syngas with a H2/CO ratio greater than 2. One notable feature of the MTG process is the first step in the process is to dehydrate methanol to produce dimethyl ether intermediate which is utilized in situ in the process to produce gasoline via subsequent catalytic reaction. One potential process to produce gasoline from syngas is to skip the methanol intermediate and synthesize dimethyl ether directly from syngas.
If the syngas is converted to dimethyl ether, the dimethyl ether can be used directly as a transportation fuel, or catalytically converted to gasoline. For example, JFE Holdings, Inc. of Japan is the assignee of a family of United States patents, including U.S. Pat. Nos. 6,800,665; 7,015,255 and 7,033,972 all of which are incorporated herein by reference, that teach the production of dimethyl ether from either the dehydration of methanol, or directly from a raw material gas comprised of CO and H2. One advantageous feature of this process is that it utilizes low ratio syngas, theoretically 1:1 H2/CO, so that syngas derived from biomass can be efficiently utilized without the need to increase the H2/CO ratio via the water-gas shift reaction. U.S. Pat. No. 7,820,867, which is also incorporated herein by reference, teaches a process for converting syngas to easily convertible oxygenates, such as methanol or methanol and dimethyl ether, then to gasoline over a zeolite type catalyst, such as ZSM-5. Because the consumption ratio of the direct DME route is 1:1, feeding this process a syngas composition above this ratio can result in a tail gas that has higher ratio than the initial syngas, making the tail gas a good candidate feed for other processes, such as the Fischer Tropsch process that requires a higher ratio syngas.
Combining the unique characteristics of these known processes in novel ways can result in unexpected increases in efficiency and yield compared to the conventional art.